In a receiver system, the channel equalizer is an essential component, improving the bit error rate (BER) by correcting the received signal for the effects of the channel. Multi-path interference, which is commonly referred to in the art simply as the multipath, presents particular problems for wireless communication systems, where a transmitted signal may arrive at the receiver over multiple transmission paths. For example, in a system having a single transmitter, the multipath transmission of a signal may occur because of signal reflection, so that the receiver receives a transmitted signal and one or more reflections of the transmitted signal. As another example, the multipath transmission of a signal may occur in a system having plural transmitters that transmit the same signal to a receiver using the same carrier frequency. A network which supports this type of transmission is typically referred to as a single frequency network (SFN).
One example of a wireless communication system wherein multi-path interference may present particular problems at the receiver is the broadcasting of a digital television (DTV) signal. In the United States, DTV broadcasting has been done using vestigial-sideband (VSB) modulation format in accordance with the Digital Television Standard, the latest edition of which published in December 2005 by the Advanced Television Systems Committee (ATSC) as Document A/53E. The ATSC-VSB data stream as specified by the ATSC has two modes. The first mode designed for terrestrial broadcasting, modulates data onto an RF data carrier frequency signal using 8 levels to represent data symbols of 3 bits each. This is known as 8 VSB. A second mode is available for higher band width cable transmissions which modulates the information using 16 levels of 4 bits each (16 VSB). Although the invention is described herein in connection with the 8 VSB mode, it is equally applicable for use 16 VSB mode. In a terrestrial DTV transmitter, the 8 VSB DTV signal is transmitted with a suppressed very-high-frequency (VHF) or ultra-high-frequency (UHF) natural carrier, with a fixed-amplitude pilot carrier corresponding in frequency and phase with the suppressed natural carrier.
As described in “ATSC Digital Television Standard” and illustrated in FIG. 1, television data is transmitted as data frames. Each data frame begins with a first data field sync segment followed by three hundred and twelve data segments, and then a second data field sync segment followed by another 312 data segments. Each segment consists of four symbols of segment sync followed by 828 symbols of data. Each data field sync segment includes a training sequence used for channel estimation in the receiver.
Receiver performance in the presence of multipath has been considered as one of the main weaknesses of the 8-VSB modulation used in the ATSC system. The introduction of the single frequency network with multiple transmitters for delivering the DTV signal brings new challenges for the ATSC equalizer design, since the delay spread of a multipath channel under such scenario becomes significantly longer than in the traditional broadcasting practice of using one high power facility to cover a wide area, where the multipath distortion are from reflected echoes. This is illustrated in FIG. 2A, which by way of example shows a simple DTV SFN having three transmitters 11, 12 and 13 with their respective coverage areas 21, 22 and 23. An ATSC DTV receiver 16 is located where all three coverage areas overlap.
FIG. 2B illustrates individual channel impulse responses 31, 32 and 33 that are associated with the signal transmissions from the transmitters 11, 12 and 13, respectively. Although all three transmitters transmit the DTV signal synchronously, the receiver 16 receives this signal from the transmitters 11, 12 and 13 with different time delays t1, t2 and t3, and with a different phase. The overall signal 35 at the receiver 16 is a sum of the signals 31, 32 and 33 from each individual transmitter.
The component of the broadcast DTV signal to which a DTV receiver synchronizes its operations is called the principal signal, and the principal signal is usually the strongest component of the broadcast DTV signal. The direct line-of-sight path from the closest transmitter is usually the path resulting in the strongest component of the broadcast DTV signal, if the direct line-of-sight path is not blocked by any intervening barrier to transmission; it is commonly referred to as the main path. Therefore, the multipath signal components of the broadcast TV signal received over other paths and from other transmitters are usually delayed with respect to the principal signal and appear as lagging multipath, signals resulting in the presence of echoes in the received signal. For the example shown in FIGS. 2A, B, the main path is the direct path between the Tx 13 and the Rx 16, the signal 31 is the main signal, and signals 32 and 33 are echoes resulting in the multi-path interference at the receiver 16 which should be equalized, or canceled by the receiver's equalizer for successful reception of the DTV signal.
It is possible however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes its operations to a longer path signal that is delayed with respect to the direct signal, there will be a leading multipath component caused by the direct signal. There may also be other leading signals caused by other reflected signals of lesser delay than the signal to which the receiver synchronizes. In the DTV art the multipath components of received signals are customarily referred to as “echoes”. The leading multipath components are referred to as “pre-echoes”, and the lagging multipath components are referred to as “post-echoes”. The echoes vary in number, amplitude and delay time from location to location and from channel to channel at a given location. FIG. 2C schematically illustrates an impulse response of such a multipath channel having a main path 45 which is not the shortest path, resulting in pre-echo 43 and post-echo 47.
For a satisfactory reception of the ATSC signal, the overall channel impulse response must fit within a time window TEQ of the ASTC equalizer used in the receiver, TEQ being the time window inside which echoes can be ‘equalized’, i.e. compensated for so as not to affect the receiver performance. For the example shown in FIGS. 2A, B, the maximum propagation time difference between all the transmitters (t3−t1) should be less than TEQ. The equalizer time window limits the allowed propagation path differences between the transmitters, thereby effectively limiting the cell size in a SFN network, and significantly increasing the total number of transmitters needed to deploy the SFN and work hours required to plan, deploy and maintain the SFN.
The amplitudes of correctable echoes are a function of their time displacement from the main signal, and are quickly reduced as the relative time delay increases; i.e. the closer together the multi-path signal components are in time, the stronger they can be in amplitude, and the further apart they are in time, the lower in level the echoes must be for the equalizer to work. Currently, best commercially available ATSC receivers employ time-domain equalizers that can only handle −10 dB echoes from −29.5 μs to 38.5 μs. As the result, the propagation path difference corresponding to different echoes can only be up to around 10 km for the ATSC receivers based on the current time domain equalizer technology. Notably, reflected echoes in urban deployment may substantially increase the propagation path differences among the echoes. It would be thus advantageous to increase the capability of the receiver to handle channels with very long delay spread.
Frequency domain equalizers can be more efficient than time-domain equalizers in handling long delay spreads, and are presently employed in wireless systems based on the Orthogonal Frequency Domain Multiplexing (OFDM), or in wireline Discrete Multitone (DMT) modulation. In these transmission techniques, each N-sample encoded symbol is prefixed with a cyclic extension to allow signal recovery using the cyclic convolution property of the discrete Fourier transform (DFT). Alternatively, the extension may be appended to the end of the signal as well. If the length of the cyclic prefix is greater than or equal to the length of the impulse response, the linear convolution of the transmitted signal with the channel becomes equivalent to a circular, or cyclic convolution (disregarding the prefix). If the channel impulse response is shorter than the length of the periodic extension, the original symbols can then be recovered by transforming the received time domain signal to the frequency domain using the DFT (implemented using, e.g., the FFT), and performing equalization using a bank of single tap frequency domain equalizer (FEQ) filters. For the cyclically extended signals, the FEQ effectively deconvolves (circularly) the signal from the transmission channel response, effectively canceling the echoes and restoring the originally transmitted signal.
However, the cyclic prefix is not available in existing signal carrier modulated broadcast and communication systems, including ATSC and GSM. In addition, if such a cyclic prefix is to be used, its length would have to be longer than the duration of the channel impulse response, which would introduce excessive redundancy and would limit the system throughput when the channel duration is long.
An object of this invention is to provide a hybrid time-frequency domain equalizer for equalizing a signal transmitted in a single frequency network receiver without a cyclic prefix.
Another object of this invention is to provide an efficient hybrid time-frequency domain equalizer for use in ATSC receivers.
Another object of this invention is to provide an iterative hybrid-domain method of channel equalizing for single-carrier signals transmitted without a cyclic prefix.